When applying heat for the purposes of cooking, canning or sterilizing an object, the process is completed once the object has attained a specific internal temperature. The internal temperature of the object is often estimated by measuring the time period over which the object is exposed to a heat source. Generally, several factors influence the heating process, such as the number of objects concurrently being heated, the initial temperature of the utensils and the object, the actual temperature of the heat source and the altitude at which heat is applied. Due to the wide variety of factors influencing the heating process, timing the process does not provide a true indication of the object's temperature condition. A more accurate determination as to when an object is fully heated is obtained by actually measuring its internal temperature.
In certain instances the internal temperature of an object may be measured during the application of heat. For example, meat thermometers and candy thermometers include probes for insertion into the substance being heated. However, with certain objects, measuring the internal temperature is not feasible. A temperature probe cannot be inserted into an object having an impenetrable outer surface, such as an egg, canned product or other sealed container. Thus, the internal temperature of the object must be estimated, as by measuring the time required for heating the object or by simulating the temperature condition within the object.
One method of determining the internal temperature of an object or container is by placing a device in thermal contact with the outer surface of the container. U.S. Pat. No. 4,805,188 discloses a time-temperature indicator having a thermochromic material placed within a cylinder with one end of the material in thermal contact with the container or closed vessel. When the temperature within the container increases to a certain level, the thermochromic material will change color indicating the temperature of the contents. The disclosed device indicates the temperature of the contents by responding to heat diffusing through the container lid. The temperature-time indicator, while providing an accurate estimate for some containers, is not particularly suitable for measuring the internal temperature of an object in which the temperature at the exterior surface does not represent the object's internal temperature. A temperature indicator for simulating the internal temperature of an object independent of the temperature at the object's surface is desirable.
An alternative method of simulating an object's internal temperature is by concurrently heating a temperature indicating device with the object. U.S. Pat. No. 4,137,769 discloses a time-temperature indicator having a thermally responsive indicator material embedded in a solid body of transparent plastic material. The plastic material preferably has a thermal diffusivity approximating that of the object being heated. The rate at which heat diffuses into the time-temperature indicator approximates the rate at which heat diffuses through the object.
During heating, the time-temperature indicator and the object will be exposed to the same conditions. Since the rate at which the object and the disclosed device are heated is approximately equal, measuring the internal temperature of the device will provide a substantially accurate estimate of the internal temperature of the object. The indicator material responds to the occurrence of a predetermined temperature within the device, thereby indicating that sufficient heat has been supplied to complete the heating process.
To insure that the indicating material is responding to heat diffusing through the plastic, in the disclosed device the indicator material is embedded within a solid body by pouring a hardenable plastic into a mold in layers. The layers of plastic bond together, forming the solid body. However, exposing the solid plastic device to a large temperature change introduces significant thermal stresses. With repeated use, the thermal stresses often cause delamination or separation of the solid body and failure of the assembly. A temperature indicator which may be repeatedly exposed to changes in temperature without developing significant thermal stresses is highly desirable.
The internal stresses introduced by repeated exposure to a temperature gradient also induce the formation of surface cracks and the emission of objectionable odors. In addition, embedding the indicating material within a solid body is a time consuming and labor intensive process which is not precisely repeatable. A temperature indicator which may be efficiently and economically manufactured is desirable. Similarly, a temperature indicating device which may be repeatedly subjected to a temperature gradient without the formation of cracks or the emission of offensive odors is desirable.